1. Field of the Invention
The present invention relates to a method and an apparatus for continuous compression forging cast steel derived from the continuous casting process. More specifically, the present invention relates to a method and an apparatus for improving the internal quality of cast steel, and, more particularly, for overcoming defects in casting such as central segregation and center porosity by performing effective compression forging at temperatures below the solidification point of the cast steel obtained by continuous casting.
2. Description of the Prior Art
In the conventional art, forming central segregation in continuously cast steel has been regarded as inevitable. This central segregation is caused by the condensation of carbon, sulfur, and phosphorous in the molten metal in the central portion of the final solidification region of the cast steel. The thus-condensed components in the molten metal appear in the form of normal segregation, causing central segregation which can deteriorate the mechanical properties in the direction of the thickness of steel plates and thus generate laminations.
Segregation in cast steel is considered unavoidable since the condensed molten steel is sucked into the leading end portion of the solidified region of the billet obtained by continuous casting and is allowed to remain as normal segregation in the thicknesswise central portion of the cast steel. The above-described suction of the condensed molten steel can be realized due to: solidification shrinkage of continuously cast steel at the front portion of the solidified region thereof; and a vacuum suction force generated due to bulging of the solidified shell.
In order to prevent central segregation, a variety of ways have been attempted, for example, electromagnetically stirring the second cooling zone. However, such attempts failed to completely eliminate semi-micro segregations and the effect obtained has not been satisfactory as yet.
Furthermore, an in-line reduction method (see "Iron and steel" Vol. 7, 1974, p. 875 to 884) has been proposed in which the cast steel is subjected to a heavy compression at the final stage of the solidification process by using a pair of rollers. However, if the portion of the cast steel containing a relatively large proportion of unsolidified layer is not sufficiently compressed, cracks can form on the interface between the solidified steel and the still molten portion. If excessive compression is applied, a strong negative segregation can be adversely generated in the central portion of the thickness of the cast steel.
In order to overcome the above-described problems, a continuous casting method has been disclosed in Japanese Patent Laid-Open No. 49-12738 in which the front end portion of the solidified region of the cast steel is subjected to a light compression by using pairs of rollers as to compensate for the volume of solidification shrink at the subject portion by this compression. Another method has been proposed in Japanese Patent Laid-Open No. 52-54623 in which an anvil is used for the purpose of having the portion in the vicinity of the region of the cast steel subjected to a heavy compression near the completion of the solidification of the cast steel. The other method has been disclosed in Japanese Patent Laid-Open No. 60-148651 in which electromagnetic stirring is performed, or ultra-sonic waves are applied to the cast steel during the solidification, and compression forging is performed near the completion of the solidification of the cast steel.
However, in a case of such light compression, even if a plurality of pairs of rollers are used to perform the light compression by several millimeters per meter, solidification shrinkages and bulgings generated in the region corresponding to the pitch between the rollers cannot be sufficiently prevented from being generated. Furthermore, if the compression is not applied to the proper position, the central segregation becomes worsened. According to the method in which an anvil is used for heavy-compressing the cast steel at its completion of the solidification, the interface between the solidified steel and the still molten portion can protect against cracking and negative segregation can be satisfactorily prevented from generation compared with the heavy compression method such as the inline-reduction method in which rollers are used, causing even the semi-macro segregation can be overcome. However, if the compression is insufficient in the region of the cast steel in which the unsolidified portion is in a great proportion, cracks can be formed on the interface between the solidified steel and the still molten portion. If the compression is performed excessively, intense negative segregation can be generated in the central portion of the cast steel. In addition, even if the portion of the cast steel in which unsolidified region is reduced is subjected to the compression, any effect cannot be obtained from this compression. Thus, the most suitable compressing conditions have not been as yet established to be performed.
Furthermore, according to the method in which the electromagnetic stirring and the compression forging or application of ultrasonic waves and the compression forging are combined, although an equiaxed crystal ratio can be increased, which assist to reduce the negative segregation, generation of negative segregation cannot be prevented simply by the increase in the equiaxed crystal ratio over the wide conditions upon the thickness of the unsolidified region, casting speed, and temperatures.
In order to overcome the above-described problems, a group including the inventor of the present invention has disclosed a method in Japanese Patent Laid-Open No. 60-82257 in which a compression-forging anvil is used for the purpose of compressing the cast steel near the completion of the solidification of the same. A patent application was applied for under U.S. Ser. No. 071,412, filed July 9, 1987, of which this application is a continuation-in-part. The present invention is based on the former application, but the claimed improvment has been added.
Hitherto, a hydraulic press system has been usually used as a continuously compression-forging machine employed in each countermeasures taken against the above-described central segregation of the continuously cast steel. For example, a method is disclosed in Japanese Patent Laid-Open No. 63-49400 in which an integrally formed frame of a "Floating Type" includes upper and lower anvils so that compression is equally applied from the upper portion by using a single hydraulic cylinder. Furthermore, a scissors method is disclosed in Japanese Patent Laid-Open No. 61-222663 in which a boosting mechanism such as lever is used.
However, the conventional devices of the hydraulic type need a great size hydraulic pressure source and pipes to be provided, causing cost required for institution and the load for maintenance becomes too large. In addition, since such device involves a relative high pressure to be used, the life of the pump and the same of the hydraulic control valve is shortened to two or three years, and the involved noise can exceed B 100 phons of loudness level. Another problem arises in that the energy loss during transference of the hydraulic pressure obtained by converting electric energy from the pump chamber to the compression forging device becomes 20 to 30%. Therefore, the above-described devices have not been satisfactory as yet in terms of the running cost.